Abstract

Conspicuous grey, clay-rich flood deposits in the predominantly olive, varved sediment in the central Santa Barbara Basin, California, were dated via varve-counting to c. AD 212, 440, 603, 1029, 1418 and 1605. The ?200-yr quasi-periodicity and the timing of individual floods match time periods of known supraregional climatic changes and the documented ?200-yr periodicity in palaeoclimate records elsewhere. The floods of c. AD 440 and AD 1418 occurred during global reorganizations of atmospheric circulation. Cold spells seem to have accompanied the floods of c. AD 1029 and AD 1605. California floods and other palaeoclimate records suggest that expressions of regional climates are modulated by solar variability with a ?200-yr periodicity. Our compilation of palaeoclimatic and Mesoamerican and South American archaeological records provides evidence that palaeoenvironmental and cultural changes were often coincident within their respective dating uncertainties and occurred in ?200-yr steps. The matching pattern supports the hypothesis that manifestations of changing climates, such as drought or flooding, served as destabilizing cofactors in transitions of precolumbian cultures. Extrapolation of the ?200-yr recurrence pattern of Santa Barbara flooding suggests that catastrophic flooding in southern California may be expected for the early part of this century.